The present embodiments relate to crest factor control. In electronic low frequency square wave lamp ballasts, such as high intensity discharge lamp ballasts, there often exists a problem caused by the parasitic resonances and time constants of the various internally connected inductive and capacitive elements. The lamp current may overshoot the desired nominal value of lamp current. FIG. 1 shows a square wave with the overshoot. FIG. 2 is an expanded view of a transition between half cycles of the square wave of FIG. 1. As the square wave transitions from one-half cycle of the lamp current to the opposite half cycle, the lamp current may overshoot the desired current level.
Lamp current overshoot is undesirable and may cause lamp electrode erosion, resulting in reduced lamp life. The sudden transient of current may cause magnetostrictive forces. These forces may cause acoustic noise in the various elements of a ballast's inductive components due to the sudden and high value changes in magnetic flux density. The current overshoot may also result in magnetostrictive movement of the internal lamp supporting structure, causing undesirable acoustic noise.
Overshoot may be limited by crest factor control. By turning a current source off for a period immediately after beginning the half cycle, the amount of overshoot may be limited. However, overshoot may continue to be a problem at different times during or after lamp warm-up or for different lamps.